I was a HP rifle competitor from about 89 to 03, after I made distinguished rifle all my shooting was done with match rifles starting with .308 win. Built a 6.5-08 panther for 600-1k to try for better barrel life over 6.5-284 along with easy to find decent brass. It worked fine. A little later one of my competitor friends Kent Reeve shortened a 6.5-08 case .080 keeping the same shoulder as a test to see if he could gain the ability to load 142 SMK's in the magazine for 300yd rapid fire while still keeping enough speed for 600yd slow. It worked well enough I did the same thing with one of my guns. Easy to make brass, just cutting off the base of the dies. I shortened and opened up the neck of a Dillon .243 power trim die for bulk of the trim work. All this was done before Remington brought out the .260 case. Having done all this mumbling, I would have jumped on DeMile's case that became the 6.5 CM if it had been available back then. Of course all this was for punching holes in paper. I did keep the last Kriger barreled 700 and stuck it in a hunting stock for my nephews to kill deer with, they loved it for stand hunting.
Even prior to your time frame, here's an interesting read of the path to one of the most dominate cartridges in Hunter Benchrest, made by shortening the 308 case:
